Locking pins are extensively used for locking buckets (blades) in landbase turbine rotors. A typical bucket support and bucket assembly includes a rotor or rotor disk having a series of slots in its outer periphery. A typical bucket includes an airfoil section, a base, and a root section. In such a configuration, the root section of the bucket slidably fits within a slot in the rotor or disk periphery. The bucket typically is locked into position on the rotor or disk by a locking pin that engages both the root of the blade and the rotor or disk. The locking pin typically is positioned in an aperture in the bottom surface of the slot. Both the locking pins and the rotor typically are made from stainless steel. A single rotor can have as many as several hundred such locking pins.
After prolonged service, locking pins occasionally become stuck in the rotor or rotor disk. Locking pins may become stuck in rotors, for example, as a result of deformation under rotor centrifugal force, surface gauging, or surface oxidation. Removing stuck locking pins from turbine rotors conventionally is a very laborious, costly, and time-consuming process. In addition to costs associated with removing the stuck locking pins, the time spent for removal also requires costly downtime of the turbine equipment. Furthermore, if a rotor is irreparably damaged during removal of a locking pin, the rotor must be replaced, resulting in considerable cost as the rotors are themselves expensive.
In one current approach, a mechanical drill is used to drill stuck locking pins, as illustrated in FIG. 1. Following mechanical drilling, a driving tool, such as a riveting gun or powder charge gun, applies force (f) to the end of a rod (5) to drive the remaining pin section (20) out from the locking pin receiving slot (15) of the rotor (10). This approach suffers from several drawbacks. One drawback is that the mechanical drilling operator needs to proceed with great caution to avoid drilling into the rotor (10) due to drill bit misalignment and deviation. The operator must carefully set up the drilling device for proper alignment, use a low drilling speed, and make multiple stops to check the aperture for possible damage to the rotor (10). This practice results in significantly long drilling times. Another drawback is that the driving force (f) exerted on the locking pin compresses the pin, which often causes swelling of the remaining pin section (20). Swelling makes removal much more difficult. It sometimes takes as much as eight hours to remove a single stuck pin (20).
There remains a need for a more efficient and cost-effective method for removing stuck locking pins from turbine rotors.
The present invention is directed to a method of removing a stuck locking pin from a turbine rotor. The locking pin comprises an elongate, generally cylindrical member having a first end, a longitudinal centerline axis, a width dimension, and a length dimension. The stuck locking pin is disposed within and extends through a locking pin receiving slot of the rotor. The method comprises forming an aperture in the first end of the locking pin along its longitudinal centerline axis. The diameter of the aperture thus-formed is less than the width dimension (diameter) of the locking pin, such that a generally cylindrical shell portion remains in the locking pin receiving slot of the turbine rotor. The formation of the aperture in the foregoing manner minimizes compressive forces in the locking pin and/or stresses between the locking pin and the rotor, so as to facilitate easy removal of the shell portion.
The shell portion of the locking pin then is removed from the locking pin receiving slot using any suitable means. Preferably, the shell portion is removed by driving a high-strength metal rod into the aperture formed in the locking pin. The high-strength metal rod can be driven by any suitable means, such as a riveting gun. The driving force stretches the shell portion, which makes it relatively easy to remove the locking pin from the rotor. This is in contrast to the technique illustrated in FIG. 1 in which the pin section is compressed by mechanical drilling, making removal more difficult.
In accordance with a preferred embodiment of the invention, a high-speed electrodischarge machining (EDM) drill is used to form the aperture in the locking pin. High-speed electrodischarge machining not only is faster and more accurate than mechanical drilling, but also imparts no more than minimal stresses between the locking pin and the rotor, e.g., stresses due to compression of the locking pin. The accuracy of the EDM drill also reduces the risk of damage to the rotor, while avoiding the significant labor associated with the current mechanical drilling technique.
The method of the present invention enables stuck locking pins to be removed from turbine rotors in a more efficient and cost-effective manner. Stuck locking pins can be removed in substantially less time than is required by currently available techniques, often in as little as one half hour or less. Consequently, the amount of costly downtime needed for removing stuck locking pins is dramatically reduced. The amount of labor and labor costs associated with removing stuck locking pins also is significantly reduced. Furthermore, the risk of irreparable damage to the rotor is reduced.